The present invention relates generally to static eliminators that use high voltage alternating current to ionize air and thereby produce mobile ions that are attracted to electrically charged articles until those articles are electrically neutral and, more specifically, to a high voltage sensor that can monitor the power supply used with static eliminators, as well as other high voltage devices.
Alternating current static eliminators have been widely used to eliminate or suppress static electricity on electronic equipment, sheets and webs of nonconductive material, plastic parts and food containers, fluid and bulk solids, and on many other uninsulated or ungrounded articles. The source of alternating current for AC static eliminators is known as a "high voltage power source," or a power unit, which is usually a current limited or ferroresonant transformer. The "high voltage power source" is supplied with alternating electric current from conventional mains, typically conditioned to be within the range of 100-440 volts alternating current and cycling at either 50 or 60 Hertz. The secondary windings of the high voltage power source operate at between 4,000 and 10,000 volts, with the current being limited to about five milliamperes to protect users from receiving severe shocks.
There are many alternating current static eliminating designs. The lower voltage designs usually have the ionizing electrodes and the passive electrodes directly connected to the alternating current power source ferroresonant transformer. The higher voltage designs use a resistance (as shown in U.S. Pat. No. 3,760,229) or capacitance (as shown in U.S. Pat. No. 3,120,626) in series with the alternating current power source to limit the electrode current and protect workers from shock or ignition of hazardous vapors. Most designs maintain the ionizing electrode at a high voltage while the remaining designs use an ionizing electrode that is maintained at ground potential (as shown in U.S. Pat. No. 3,369,152). Other designs have both electrodes isolated from ground (as shown in U.S. Pat. Nos. 4,053,770 and 5,307,234) that use the voltage difference between electrodes to drive the ionization process. The above designs can be incorporated into "single point ionizers," "static bars" that are linear rays of single point ionizers, blow-off guns and nozzles (as shown in U.S. Pat. Nos. 3,156,847 and 3,179,849), ionizer fitted air movers (as shown in U.S. Pat. No. 4,440,553), or the like. The variety of electrode designs and operating voltages requires that a high voltage sensor be capable of use with the various usable designs and electrode voltages.
Typically, the high voltage power source on a static eliminator is provided with an indicator light, which is intended to indicate that the alternating current static eliminator is "on." In other words, the indicator light indicates that the static eliminator is being supplied with electric current from the mains. The problem with this approach is that it reveals only that power is connected to the primary windings of the high voltage transformer. This can be problematic as it is the voltage across the secondary windings of the transformer that drives the ionization from the electrodes. Thus, the sensor can generate misleading information when there is a failure in the secondary winding or in the electrode system connected to that winding. Accordingly, when the sensor is attached to the primary windings, it is possible that even though the indicator light is in the "on" condition, the ionizing apparatus may be essentially non-functioning. Thus, ideally, a high voltage sensor should be able to establish whether the output (i.e., secondary winding) of the transformer is shorted out by the failure of the ionizer, the high voltage wire, or the transformer insulation systems.
An alternating current static eliminator uses an oscillating voltage that is the same at all ionizing electrode sets and along all connecting wires to the alternating current source. The conductor system is an equipotential system. The amplitude of the voltage depends upon the load and ionizer design. In monitoring the potential for ionization, it is therefore sufficient to use one sensor to monitor the entire ionizer system. Accordingly, the sensor should be placed at the most convenient location on the high voltage power source for monitoring the static eliminator system. Thus, ideally, the high voltage sensor should be designed to be sufficiently flexible with respect to being positionable at various locations along the combination high voltage power source and static eliminator system, including being positionable along the wires connecting the alternating current source with the AC static eliminators. This allows the sensor to be placed at the most convenient location for monitoring the system.
The insulation system of a static eliminator, including cables, is normally exposed to hostile industrial environments and consequently has a finite life. As such, failure detection circuits are important to alert equipment operators when static elimination is no longer occurring. Accordingly, an ideal high voltage sensor should match and preferably exceed the durability of the other components of the ionizer system. Additionally, the high voltage sensor should be replaceable separately from the other components of the ionizer system.
High voltage sensors can be generally classified as either directly coupled or capacitively coupled. Directly coupled high voltage sensors are attached directly between the high voltage terminals. Some directly coupled high voltage sensors operate from current drawn from the secondary windings of the high voltage power source, while others rely on current drawn from the mains to drive the electrical circuitry. The use of power from the mains permits brighter visual indications of failure, allows the use of relays to operate control circuits, and makes possible the use more sophisticated provisions for interpretation of failure modes. The sensing and relay circuits of the mains-powered circuits are generally located inside the high voltage power source.
Typical direct coupled sensors, such as the SK-4/7 from Simco Japan, include a failure detection circuit to turn "off" the high voltage and inform the operator when there is a high voltage electrode or cable failure. The sensing circuits are mounted inside the high voltage high voltage power source and include a voltage divider across the output of the transformer. Simco Japan manufactures an optional, external monitor that uses red and green lights and a buzzer with its failure detection circuit. The dual-phase high voltage power source with a trip circuit manufactured by Simco USA demonstrates another approach for designing sensing circuits which determine the secondary voltage from the cap-coil of the ferroresonant transformer. The detection signal is used to operate a relay that turns "on" a flashing light and disconnects power to the transformer. The circuit can only be used with ferroresonant transformers and must be part of the high voltage power source. The threshold for voltage trip-out, and delay before the full trip-out, can be adjusted, but are fixed in practice. The trip circuit is also commercially available as a stand-alone unit or as an integral part of a single phase high voltage power source. Sophisticated sensors (such as that shown in U.S. Pat. No. 3,584,258) discriminate between a streamer corona and the non-carbonizing sparks within the dielectric, high leakage currents, and arcs. Other similar circuits are available in the industry.
Capacitively coupled sensors obtain signals that are used to detect high voltage through the capacitance of the insulation system. This approach has an advantage in some applications where the sensor is to be incorporated in the capacitively coupled ionizer. Such ionizers are often compact in design and thus require that the insulation system remain unbroken while securing measurements. In these compact ionizers, breaks in the insulation system become failure points for the electrode system. Additionally, there are other disadvantages with capacitively coupled sensors. The current from the capacitive sensor is typically on the order of ten microamperes or less. The small amount of current makes it difficult to power a sensor sufficiently to generate a sufficiently visible indication of electrode failure. Furthermore, to use capacitance to detect a voltage requires that a time varying current be present in the wire. Unfortunately, electrical sparking during electrode or insulation system failure in any part of the ionizer, cable, or high voltage power source system will send high voltage transients through the capacitively coupled indicator circuits which will interfere with the signal detected by the sensor. Sophisticated transient limiting circuits are needed to prevent the burn out of electronics circuits and capacitive sensors, such as electroluninescent devices.
Some capacitively coupled sensors use capacitive-coupling with an operational amplifier (which may be either linear or analog) and a comparator to operate a relay. Outputs from the circuit include a light emitting diode and terminal for a direct current signal between twelve and fifteen volts (Haug).
Circuits powered from the mains, whether directly or capacitively coupled, are generally too expensive for the majority of commercial applications. For this reason, a demand exists for circuits that are separately manufactured which can indicate the presence of a high voltage. The sensor's circuit construction must enable a low cost, visible indication of the detected high voltage, and have a component life which exceeds that of the other components of the alternating current static eliminator system.
A better indication of high voltage from capacitively coupled sensors can be obtained using a spark gap to store energy and spark over through a sensor. Such a principle is incorporated in Simco's Static Bar Checker. The use of a spark gap is not feasible for continuous use because of erosion of the gap and attendant radio frequency interference caused by the sparking.
Another approach incorporates rectifying diodes and a storage capacitor to produce a bright, blinking indication of high voltage. The connection to the static bar is accomplished by either a direct ring placement around the inner bar (such as a high voltage cable insulation system) or by connection to a pin of the capacitively coupled bar. Such a circuit is illustrated in Simco drawings 4220054 (Jul. 20, 1982) and the physical arrangement of the components is shown in Simco drawing 4101579 (Jun. 22, 1984). This technique was also implemented by Simco in their industrial ionizing air blowers (e.g., the AS-20 air blower). Space limitations in the more compact ionizers prevent the connection to a pin of the capacitively coupled bar. Circuits of this type have been used for hot line detection, for high voltage switch gear and transmission lines (as shown in U.S. Pat. Nos. 3,970,932; 3,991,367; 4,259,545; 4,794,329; 4,814,933; 5,051,733; and 5,065,142) and test probes (as shown in U.S. Pat. Nos. 4,152,643 and 4,870, 343).
Alternatively, a series circuit including a capacitor, resistor, and neon light can be placed between the high voltage wires. The size of this capacitor is usually five to ten times greater than the capacitance of the sensor in capacitively coupled sensors. The use of a discreet capacitor in the circuit is a lower cost method for melding the direct and capacitively coupled designs for high voltage sensors. Simco has incorporated this type of circuit in several of their ionizing air blowers (such as the Aerostat XC, Aerostat PC, and the Guardian Overhead Aerostat) and an ionizing airgun (such as the Top Gun power unit).
Simco has attempted to use various sensors over the years that were directly incorporated into the actual static bar component for web and sheet handling applications, but discovered that such direct mounting of sensors onto static elimination bars creates many difficulties. The first difficulty is that each different model static bar requires a unique design for the sensor that is incorporated therein. Additionally, static bars are often placed in locations where the light which was mounted thereon could not be seen. Another difficulty was caused by neon light sensors sometimes yielding premature static bar failure indications because of the limited space that is available in the bar construction. Furthermore, the physical size of the static bars which incorporated sensors were less desirable to customers. Still another problem with incorporating sensors with static bars is that the use of multiple sensors when using multiple bars tends to be inconvenient. This is especially irksome when one conveniently placed sensor could be used to determine if the proper amount of power is being supplied to each of the bars.
What is needed, but so far not provided by the prior art, is a high voltage sensor that monitors the voltage between a high voltage power source and a static eliminator; that monitors the output of the secondary winding and thus, is not shorted out by failures of the ionizer, the high voltage wires, or the transformer insulation systems; that is a separate component from the high voltage power source and that is insertable therein; that can be used with systems having various cable designs and electrode voltages, including ionizers where neither electrode is at ground potential; that can be positioned at the high voltage power source or at any convenient location along the wires connecting the high voltage power source to the static eliminators; that has a construction which exceeds the durability of other components of the ionizer system; and that is manufacturable at a relatively low cost.